1. Field of the Invention
The present invention relates to a method and an apparatus for aligning three-dimensional shape data obtained by performing a three-dimensional measurement of an object, by using target marks provided on the object.
2. Description of the Related Art
A three-dimensional measurement in a noncontact manner is frequently performed to obtain three-dimensional shape data of objects. Examples of the three-dimensional measurement include passive types such as a lens focus method and a stereo image method and active types such as an optical radar method and a light projection method. According to the stereo image method, the object is photographed from a plurality of different positions, for example, by a camera, and from the obtained images, the three-dimensional coordinates of each point on the object are calculated based on the principle of triangulation. This method is also called photogrammetry, because the three-dimensional coordinates are obtained by photographing the object. According to the light projection method, detection light is projected onto the object, and the reflected light from the object is received by an image sensor. A method using slit light as the detection light is a slit light projection method (also called a light-section method). According to the slit light projection method, slit light is deflected to optically scan the object, and from the degree of deformation of the slit light based on the surface configuration of the object, the three-dimensional coordinates of each point on the object are calculated based on the principle of triangulation.
To obtain complete three-dimensional shape data of one object, it is necessary to perform the three-dimensional measurement of the object from different positions a plurality of number of times and combine the obtained pieces of three-dimensional shape data into one piece. In particular, with active-type three-dimensional measurement instruments (three-dimensional digitizers), since the viewing angle is limited, there are cases where the entire object does not fall within the visual field of one measurement. For this reason, it has conventionally been performed to perform the measurement of the object part by part and combine the obtained pieces of three-dimensional shape data into one three-dimensional model.
In combining a plurality of pieces of three-dimensional shape data, it is necessary to align them. For the alignment of the three-dimensional shape data, it is performed to paste a multiplicity of target marks on the object and perform alignment based on the target coordinates which are the three-dimensional coordinate positions of the target marks.
However, in a case where the alignment is performed based on only the target coordinates like the conventional method, when there are a plurality of target coordinates in a similar positional relationship, there are cases where which target coordinates are correspondent cannot be found.
For example, in a case where six target marks TM are provided on an object Q as shown in FIG. 14(A), when the triangles of two groups A and B each including three target marks TM surrounded by a chain line are similar to each other, these cannot be distinguished from each other. In this example, as shown in FIG. 14 (B), the two triangles of the groups A and B are mirror symmetric and the correspondences among the target marks TM coincide with each other, so that it is difficult to distinguish these from each other. Consequently, there is a possibility that the pieces of three-dimensional shape data are erroneously aligned.
As mentioned above, the conventional method has a problem that the robustness in determining the correspondence among the target coordinates is low. To solve this problem, it is considered to irregularly paste the target marks TM when providing them on the object Q or increase the number of target marks TM. However, since irregularly pasting the target marks TM is difficult to quantify and is, in a large measure, know-how, depending only on it is a problem. When the number of target marks TM is increased, since the area of the object hidden by the target marks TM is increased, a new problem arises that the quality of the measured three-dimensional shape data is degraded.